The present invention relates to a method for fabricating thin film layered structures containing lithium compounds by vapour deposition.
The deposition of materials in thin film form is of great interest owing to the many applications of thin films, and a range of different deposition techniques are known. Various of the techniques are more or less suitable for particular materials, and the quality, composition and properties of the thin film produced typically depends greatly on the process used for its formation. Consequently, much research is devoted to developing deposition processes that can produce thin films appropriate for specific applications.
An important application of thin film materials is in solid state thin film cells or batteries, such as lithium ion cells. Such batteries are composed of at least three components. Two active electrodes (the anode and the cathode) are separated by an electrolyte. Each of these components is formed as a thin film, deposited in sequence on a supporting substrate. Additional components such as current collectors, interface modifiers and encapsulations may also be provided. In manufacture, the components may be deposited in the order of cathode current collector, cathode, electrolyte, anode, anode current collector and encapsulation, for example. The structure of these batteries places additional burdens on the deposition processes of the individual components, since the fabrication of a particular layer should not have a detrimental effect on any layers already deposited.
In the lithium ion battery example, the anode and the cathode are capable of reversibly storing lithium. Other requirements of the anode and cathode materials are high gravimetric and volumetric storage capacities which can be achieved from a low mass and volume of material, while the number of lithium ions stored per unit should be as high as possible. The materials should also exhibit acceptable electronic and ionic conduction so that ions and electrons can move through the electrodes during the battery charge and discharge process.
Otherwise, the anode, cathode and electrolyte require different properties. The cathode should present reversible lithium intercalation at high potentials, while the anode should present reversible lithium intercalation at low potentials.
The electrolyte physically separates the anode and cathode, so it must have extremely low electrical conductivity to prevent short circuiting of the battery. However, to enable reasonable charge and discharge properties the ionic conductivity of the material must be as high as possible. Furthermore the material must be stable during the cycling process and not react with either the cathode or the anode.
The manufacture of solid state batteries poses a range of challenges. In particular, reliable and efficient techniques for producing materials suitable for use as cathodes are of great interest. For several popular cathode materials, the cathode layer of the battery is required to have a crystalline structure to provide the required properties outlined above. However, depositing a quality crystalline cathode layer in a way which is compatible with subsequent steps in the manufacturing and processing of the complete battery is often problematic. Electrolyte materials are often preferred or required to be amorphous, and further challenges have been the identification of solid electrolytes with sufficiently high ionic conductivity, low electronic conductivity, and low mechanical stress resulting from the required electrochemical cycling and reproducible high yield production methods.
Many different methods of depositing the components of thin film batteries are known. Synthetic routes to thin films, which are generally referred to using the umbrella term of ‘physical vapour deposition’ include: pulsed laser deposition (Tang, S. B., et al., J. Solid State Chem., 179(12), (2006), 3831-3838), flash evaporation (Julien, C. M. and G. A. Nazri, Chapter 4. Materials for electrolyte: Thin Films, in Solid State Batteries: Materials Design and Optimization (1994)), sputtering, and thermal evaporation.
Of these sputtering is the most widespread deposition technique. In this method a target of a particular composition is sputtered using plasma formed over the target and the resulting vapour condenses on a substrate to form the thin film. Sputtering involves the deposition of materials directly from a target. The product of the sputter varies and may include dimers, trimers or higher order particles (Thornton, J. A. and J. E. Greene, Sputter Deposition Processes, in Handbook of Deposition Technologies for films and coatings, R. F. Bunshah, Editor 1994, Noyes Publications). The deposition rate, composition, morphology, crystallinity and performance of thin films are determined by a complex relationship with the sputtering parameters used.
Sputtering can be disadvantageous in that it is difficult to predict the effect of the various sputtering parameters on the characteristics and performance of the deposited material. In part this is due to the confounding of deposition parameters with film characteristics. The deposition rate, composition, morphology, crystallinity and performance of sputtered thin films are determined by a complex relationship with the sputtering parameters used. Hence, it can be very difficult to alter the individual parameters of the sputtered film (for example, concentration of a single element, deposition rate or crystallinity) without affecting other properties of the film. This results in difficulties in achieving a composition of interest, which makes optimisation of the film and therefore of the properties of any intended battery or other thin film device extremely problematic.
The technique of pulsed laser deposition (PLD) shares many properties with sputtering, due to the use of a compositionally unique target and the use of high energies. In addition, this route often yields very rough samples, which is also a problem with sputtering. The surface morphology of LiMn1.5Ni0.5O4 thin films prepared by PLD has been noted to be rough, with formation of grains with sizes from 30 nm upwards dependent on the temperature of the deposition (Wang et al. Electrochimica Acta 102 (2013) 416-422). Surface roughness is highly undesirable in layered thin film structures such as a solid state battery, where any large protruding features could extend well into, or even through, an adjacent layer.
Deposition of thin films from thermal evaporation of compound sources has been demonstrated in the synthesis of battery components for materials such as LiMn2O4 (LMO, lithium manganese oxide) and B2O3—Li2O (Julien, C. M. and G. A. Nazri, Chapter 4. Materials for electrolyte: Thin Films, in Solid State Batteries: Materials Design and Optimization (1994)). In this instance the particle energies are much lower than those encountered in sputtering, which can inhibit cluster formation, reduce surface roughness and provide smooth, undamaged surfaces. However, problems such as variation in composition between source and resultant thin film are common to all routes which begin with compound evaporation targets (including sputtering and PLD). Furthermore it has been noted that a relationship between substrate temperature and the composition of the deposited film exists once again resulting in difficulties in optimising material performance due to a confounding of parameters.
An alternative is thermal evaporation directly from the elements, but this is uncommon. Julien and Nazri (Chapter 4. Materials for electrolyte: Thin Films, in Solid State Batteries: Materials Design and Optimization (1994)) allude to an attempt to synthesize B2O3-xLi2O-yLinX (X═I, Cl, SO4 and n=1, 2) directly from the elements, but no results are reported and the authors comment that “the difficulties in implementing this technique stay in enhancing the oxygen pumping, avoiding the high oxygen reactivity with the heated parts of the system, and making available an oxygen monoatomic source in order to enhance oxygen reactions on the surface”.
Previously, the present inventors have demonstrated the synthesis of phosphorous-containing materials, suitable for some thin film battery components, directly from the constituent elements (WO 2013/011326; WO 2013/011327). However, a complexity in this process is the use of a cracker to break down phosphorous so as to enable the formation of phosphates. The synthesis of cathode (lithium iron phosphate (LFP)—example 5, lithium manganese phosphate (LMP)—example 7) and electrolyte materials (Li3PO4—example 1 and nitrogen doped Li3PO4—example 6) is disclosed. The deposited materials are amorphous, so annealing is used to crystallise the cathode materials, at temperatures of 500° C. and 600° C. for LFP and LMP respectively. Although this work demonstrates two of the three basic building blocks for producing a thin film cell, it does not demonstrate an operational cell. Furthermore, the ionic conductivities demonstrated in this work are too low to enable a cell to function correctly at room temperature. It is widely known that a conductivity of 1×10−6 S cm−1 at room temperature is required for satisfactory performance, but this was not demonstrated.
State of the art cathode materials such as LiMn2O4 (LMO) and LiMn1.5Ni0.5O4 (LMNO) are required to be in the crystalline state to achieve optimum ionic conductivity. This can be achieved by either a low temperature deposition followed by post annealing, or depositing at high substrate temperatures. Cathodes deposited at low substrate temperatures (below 250° C.) are often insufficiently crystalline, resulting in reduced performance. Therefore, a further crystallisation step is required; this is typically achieved by a post-deposition, high temperature anneal, although this step can result in a lithium deficiency in the film (Singh et al. J. Power Sources 97-98 (2001), 826-831). Samples of LMO deposited by RF-sputtering on to a heated substrate have been found to have an initial crystallization temperature of 250° C., while films grown using a substrate temperature below 200° C. exhibited a broad and diffuse XRD pattern, consistent with materials that are X-ray amorphous (Jayanth et al. Appl Nanosci. (2012) 2:401-407).
Crystalline LiNi0.5Mn1.5O4 (LMNO) thin films have been deposited using PLD at higher substrate temperatures, between 550 and 750° C. followed by a post anneal for one hour at the same temperature (Wang et al. Electrochimica Acta 102 (2013) 416-422). Using high substrate temperatures such as these has certain disadvantages. Loss of lithium is more significant at higher temperatures, and there is the possibility of cross contamination due to diffusion of materials between the deposited film and the substrate, which limits the possible substrates available for use.
The process of annealing, which involves exposing a material to a high temperature to crystallise it, is potentially applicable whenever a deposited material is insufficiently crystalline. The temperature required will depend on the material, but is typically at least 500° C. and may be significantly higher, and the resulting crystalline layer may be of poor quality. As an example, the cathode material lithium manganese nickel oxide (LMNO; LiMn1.5Ni0.5O4) has been crystallised by annealing sputtered thin films, but the resulting layers are polycrystalline and composed of grains with diameters between 50-150 nm. The films have also been shown to contain a number of impurity phases (Baggetto et al., J Power Sources 211 (2012) 108-118).
Also, annealing is an undesirable complexity, and is particularly problematic in the manufacture of a complete battery or other layered device. As discussed, solid state batteries based on state of the art materials require crystalline electrodes (such as cathodes made from LiCoO2, LiMnO4, LiMn1.5Ni0.5O4), and amorphous electrolytes (such as LiPON). This requirement means it has generally been necessary to anneal the cathode prior to the deposition of the electrolyte layer, to avoid crystallisation of the electrolyte. This step requires both time and energy to provide sufficient crystallisation. Furthermore, one or more high temperature processing steps (e.g. LiMn1.5Ni0.5O4: 550-750° C. for PLD (Wang, Y., et al., Electrochimica Acta, (2013), 102(0), 416-422) and multiple anneals at 750-800° C. for sol-gel and solid state synthesis (Zhong, Q., et al., J. Electrochem. Soc., (1997), 144(1), 205-213) are typically required, thereby limiting components such as the substrate to those compatible with such high temperature processing. Additionally such a process prevents the deposition of an additional cell onto an existing one to produce a stack of cells, since any amorphous electrolyte already deposited must also be annealed, causing crystallisation. Such crystallisation is known to cause a dramatic reduction in the ionic conductivity of the state of the art electrolyte, LiPON. The conductivity of crystalline LiPON is known to be by seven orders of magnitude lower than that of the amorphous material (Bates et al. Synthesis, Crystal Structure, and Ionic Conductivity of a Polycrystalline Lithium Phosphorus Oxynitride with the γ-Li3PO4 Structure. Journal of Solid State Chemistry 1995, 115, (2), 313-323). Further problems associated with such high temperature processing include delamination and cracking of individual layers.
Techniques have been proposed to address these problems. For example, it has been shown that by providing a focused beam of ions it is possible to deposit crystalline films in-situ (WO 2001/73883). In this case adatoms (defined in this case as a particle, molecule or ion of material that has not yet been formed into a structure or film) of a cathode material were deposited onto the substrate while a second flux of ions supplied energy to the cathode material. A flux of the second material provided energy to the first material to assist in the growth of a desirable crystalline material. This has been demonstrated for LiCoO2 as a cathode. In the case of LiCoO2 a beam of O2 ions was utilized. This beam of oxygen is noted to have two functions, both controlling the crystallization and the stoichiometry of LiCoO2 (EP 1,305,838). The beam of ions addresses three problems encountered in the preparation of thin films—that of compositional control, preparation of dense films and crystallisation of the resultant films. However, the ion beam is a complicating aspect of the process.
Turning to the electrolyte component of the battery, both crystalline and non-crystalline (amorphous) electrolyte materials have been considered. Crystalline materials such as lithium lanthanum titanate (LLTO), thio-LISICON, NASICON-type (Li1+x+yAlx(Ti,Ge)2−xSiyP3−yO12), and Li10GeP2S12 generally exhibit excellent ionic conductivity (for example up to 1.2×10−2 S cm−1 in the case of Li10GeP2S12) so appear to be good candidates for electrolytes. However, these materials present problems when applied to battery systems. In the case of the oxides (LLTO, thio-LISICON and NASICON-type) the transition metals within the electrolyte are prone to reduction which causes the material to exhibit electronic conductivity and thus short circuit the battery. The sulphide systems, such as Li10GeP2S12, present extremely high conductivities but are prone to decomposition when exposed to air and water, causing the release of toxic H2S and a deterioration in performance. Furthermore, both oxide and sulphide crystalline electrolytes require extremely high processing temperatures. For these reasons crystalline electrolytes have not been utilised in commercial thin film battery systems.
Amorphous electrolytes such as lithium phosphorous oxynitride (LiPON), lithium silicate and lithium borosilicates exhibit much lower levels of ionic conductivity. Although the optimum conductivity of these materials is at least two orders of magnitude lower than that of the crystalline materials, this has been determined to be acceptable if the electrolyte is less than 1×10−6 m thick (Julien, C. M.; Nazri, G. A., Chapter 1. Design and Optimisation of Solid State Batteries. In Solid State Batteries: Materials Design and Optimization, 1994). LiPON has an acceptable ionic conductivity of 3×10−6 S cm−1 and has been shown to be stable in air and when cycled against lithium. For these reasons, coupled with its ease of manufacture, it has been widely adopted in the first generation of solid state batteries (Bates, J. B.; Gruzalski, G. R.; Dudney, N. J.; Luck, C. F.; Yu, X., Rechargeable Thin Film Lithium Batteries. Oak Ridge National Lab and Solid State Ionics 1993; U.S. Pat. No. 5,338,625). The amorphous nature of these electrolytes is critical to their performance; crystalline LiPON has an ionic conductivity seven orders of magnitude lower than the amorphous material. However, the amorphous LiPON will crystallise at temperatures lower than those typically needed to anneal cathode materials such as LMO (LiMn2O4, lithium manganese oxide) and LMNO (LiMn1.5Ni0.5O4, lithium manganese nickel oxide) using standard synthesis techniques, thereby complicating the manufacture of a thin film battery comprising these materials.
Hence, amorphous electrolytes are of great interest. An alternative to LiPON is amorphous lithium borosilicate. Amorphous lithium borosilicate materials with ionic conductivity comparable to LiPON have been produced, but by methods requiring rapid quenching (Tatsumisago, M.; Machida, N.; Minami, T., Mixed Anion Effect in Conductivity of Rapidly Quenched Li4SiO4—Li3BO3 Glasses. Yogyo-Kyokai-Shi 1987, 95, (2), 197-201). This synthetic method gives rise to irregular ‘splats’ of glass, which are not suitable for processing into thin film batteries. Synthesis by sputtering of similar compositions has been attempted in thin films, but these were not successful, resulting in materials with significantly reduced conductivities when compared to the rapidly quenched glass (Machida, N.; Tatsumisago, M.; Minami, T., Preparation of amorphous films in the systems Li2O2 and Li2O—B2O3-SiO2 by RF-sputtering and their ionic conductivity. Yogyo-Kyokai-Shi 1987, 95, (1), 135-7).
The inventors' previous work (WO 2013/011326; WO 2013/011327) showing the deposition of thin films of phosphate materials from constituent elements suggests that the technique might be possible for other materials, such as lithium borosilicate. However, it has been found that following the method described for phosphates while using the constituent elements of lithium borosilicate does not produce the desired thin film material. The elements fail to react on the substrate in the manner expected from the phosphate work so that the required compound is not created. Using molecular oxygen instead of atomic oxygen overcomes this problem, however the lithium borosilicate phase of interest is only achieved when the substrate temperature is low (room temperature). Deposition of lithium borosilicate utilising molecular oxygen at elevated temperatures results in the formation of crystalline LiPt7 when deposited onto platinum substrates, an unwanted phase. Furthermore, Raman spectra of the materials deposited at elevated temperatures do not exhibit the bands associated with the lithium borosilicate phase of interest. Also, the resulting amorphous lithium borosilicate will crystallise when subjected to the elevated temperatures required for producing and processing other components of a battery, or if the battery is for use at high temperature, thereby destroying its electrolyte qualities.
The efforts required to overcome these many difficulties in the various deposition processes and the complexities involved in developing new materials mean that the vast majority of thin film batteries are limited to using LiPON as an electrolyte, deposited as a thin film by sputtering.
Clearly, improved methods of making thin films of other electrolyte materials are desirable, plus there is a need for an improved and simplified method of depositing crystalline materials suitable for use as electrodes in thin film batteries. Additionally, incompatibilities between methods for forming crystalline and amorphous materials need to be addressed so that the fabrication and performance of thin film batteries and other layered thin film devices can be improved.